Are things more expensive to buy?
It's been a while since I played Fallout 3, but I don't think the prices are much higher, if at all. I think the difference is that vendors have a lot more caps this time around. In order to sell all your loot from a typical scavving run in FO3, you might have to use two or three different vendors, or wait around for that vendor's inventory to restock (which, of course, is a bad idea in this game if you're playing hardcoe). In this game, most vendors have at least 500 caps on them, and usually it's quite a bit more than that.
So it's not that your items are selling for more money (I think weapons and armour sell for quite a bit less, actually, but that might just be because my Barter is still low), but rather that you can sell them all more easily.
EDIT: Actually, come to think of it, the items are definitely cheaper. I remember I always used to collect mercenary armour and repair it up while I was adventuring, but when I did that in Primm today, even at 100 CND it was worth less than 100 caps, and that's regardless of Barter (I wound up selling it for something like 28).